Molded manhole unit

ABSTRACT

A corrosion resistant manhole unit having a rotationally molded plastic manifold forming a hollow chamber for providing access to sewer pipes to be connected to the manifold. A manifold inlet connector molded into the manifold for connecting the manifold to an upstream conduit and allowing an inflow of fluid to the hollow chamber. A manifold outlet connector molded into the manifold for connecting the manifold to a downstream conduit so that fluid flows out of the hollow chamber. A manifold riser molded atop the manifold having an open top forming an aperture for accessing the hollow chamber. The manifold riser extending upward from the manifold towards ground level for allowing equipment and personnel to access the hollow chamber through the aperture. A watertight removable riser cap is included for sealing the top of the riser to close and prevent fluid and debris from entering or leaving the manifold.

FIELD OF THE INVENTION

[0001] The present invention relates to manhole structures, and the like, for gaining access to underground facilities such as sewers, pipelines, valves, etc, in order to inspection, repair, and maintenance the facilities. More particularly, the present invention relates to a single-piece, rotationally molded polyethylene manhole unit that incorporates a bell inlet connector and spigot outlet connector for quick and reliable watertight seals without external rubber sleeve seals.

BACKGROUND OF THE INVENTION

[0002] In the construction of underground water and sewer facilities, it is a standard practice to place a series of manholes at regular intervals to provide access to the pipelines for inspection, repair, and maintenance. Most often, these manholes are constructed of concrete shaped into a cone or similar configuration of sufficient size to allow cleaning equipment, or even an entire person, to enter the manhole.

[0003] Typically, the concrete manholes are made from prefabricated sections, which are assembled at the plant or on site. However, the assembly of these manholes requires a substantial mount of time, labor and equipment due to the substantial weight of the preformed concrete sections. Additionally, because the concrete is not easily machines or prefabricated with precision, it is difficult to fit the concrete manholes to existing pipelines without leaks. Unfortunately, leakage represents a significant problem to closed water and sewer systems because silt and other contaminates find their way into the manholes and pipes, eventually building-up and interfere in the operation of the sewer and water systems by causing the systems to overflow or produce a surcharge of wastewater at the treatment facilities. In order to prevent leaks, it is common to use flexible rubber boots or sleeves attached around the outside of the pipe connections. Over time, however, because these rubber boot seals are disposed around the outside of the pipes, they are overly exposed to the elements and are easily corroded, which ultimately causes the seals to fail and cause leaks. Additionally, the concrete manholes themselves crack and split due to environmental conditions and corrosive agents in the system that disintegrate the concrete, allowing additional contaminants into the system.

[0004] In order to combat the corrosive agents affecting manholes, U.S. Pat. No. 3,745,738 discloses a corrosion resistant manhole formed from concrete and lined with glass-reinforced polyester. The manhole is formed through a labor intensive, time consuming and expensive process of pouring concrete around the glass-reinforced polyester lining and allowing the concrete to harden. Given today's extremely strong and durable advanced composite materials, it is unnecessary to perform such a complicated operation to provide a strong and corrosion resistant manhole. Additionally, there is no disclosure for improving the method of sealing the sewer pipes to the manhole to prevent corrosion of the seals. Accordingly, a more cost effective manhole with a simplified installation process that provides a durable watertight seal is needed.

[0005] U.S. Pat. No. 5,361,799 sought to improve on the manhole by providing a watertight wastewater access fitting formed from preformed plastic, which is inexpensive and essentially impervious to corrosive chemical agents found in sewer systems. However, this manhole still uses the external rubber sleeve seals around the exterior of the sewer pipes and fails to address the problem of preventing leaks caused by corrosion of the seals. Additionally, the manhole fails to include any reinforcing structures to strengthen the structural integrity of the manhole sufficient to expose the manhole to the pressures applied deep underground and therefore is of limited use and unsafe for persons to enter. Accordingly, there is a need for a watertight reinforced corrosion resistant manhole that does not require an external rubber seal to connect pipes and can maintain its structural integrity when buried deep underground.

[0006] Therefore, it is an object of the present invention to provide a molded plastic manhole unit having reinforcing structural members for strengthening the manhole to withstand external pressures applied against the manhole when buried underground.

[0007] It is an object of the present invention to provide a molded plastic manhole unit having watertight seals protected from overexposure to corrosive agents surrounding the pipes so that the integrity of the seals is maintained to prevent overflow and surcharge caused by leaks.

[0008] It is an object of the present invention to provide a molded plastic manhole unit resistant to the corrosive effects of acids and other corrosive chemical agents present in sewage system pipelines.

[0009] It is an object of the present invention to provide a molded plastic manhole unit incorporating a riser extending to ground level to provide access to the manhole interior for the inspection, maintenance and repair of the sewer pipes.

SUMMARY OF THE INVENTION

[0010] The above objectives are accomplished according to the present invention by providing a single-piece corrosion resistant manhole unit for placement underground to connect with a conduit system in a watertight manner and provide access to the conduits for inspection, maintenance, and repair.

[0011] The manhole comprises a rotationally molded polyethylene manifold forming a hollow chamber for providing access to conduits to be connected to the manifold. In the preferred embodiment, the manifold includes a plurality of lateral manifold extensions molded into the manifold for connecting to the conduits to facilitate the flow of fluids through the hollow chamber. A manifold inlet connector is included in the manifold extensions for connecting the manifold to an upstream conduit spigot for allowing an inflow of fluid to the hollow chamber. A manifold outlet connector is included in the manifold extensions for connecting the manifold to a downstream conduit bell for allowing fluid to flow out of the hollow chamber. Additionally, a manifold riser is molded atop the manifold and extends upward from the manifold towards ground level for allowing equipment and personnel to access the hollow chamber. A removable riser cap is included for sealing the top of the riser to close and prevent fluid and debris from entering or leaving the manifold. As a results, a cost effective, lightweight, and corrosion resistant manhole is provided that can be quickly and easily installed with watertight seals to reduce the problems of overflow and surcharge associated with leaking conduits.

[0012] In a further advantageous embodiment, the manhole unit includes manifold reinforcing elements formed into the manifold during the rotational molding process for increasing the structural integrity of the manifold. Preferably, the manifold supports are a plurality of reinforcing ribs protruding from the sides and bottom of the manifold. The ribs reinforce the hollow chamber to withstand external pressures applied to the manifold when buried underground.

[0013] In the preferred embodiment, the manhole unit includes a fluid channel molded into the manifold between the manifold inlet connector and manifold outlet connector for channeling fluid through the hollow chamber. The fluid channel is declined to form a downward gradient through the hollow chamber from the manifold inlet connector to the manifold outlet connector so that fluid is caused to flow through the hollow chamber in the fluid channel and continue through the conduit system.

[0014] Preferably, the manifold inlet connector includes an annular seal for sealing between the manifold inlet connector and upstream conduit spigot. The annular seal conforms to surface variations in the shape of the conduit spigot to form a watertight connection. Advantageously, the annular seal is carried on an interior side of the manifold inlet connector so that the annular seal is protected from exposure to environmental corrosive agents surrounding the conduits.

[0015] In the preferred embodiment, the manhole unit includes a riser extension for connecting with the manifold riser. The riser extension receives the manifold riser to extending the height of the manifold riser to ground level when buried underground so that access to the hollow chamber can be maintained at ground level when the manifold riser is of insufficient height. A sealing ring is carried by the riser extension for forming a watertight connection between the manifold riser and the riser extension. The sealing ring is carried on an interior side of the riser extension so that it is not exposed to environmental corrosive agents surrounding the outside of the conduits.

[0016] Preferably, an adaptive seal is also carried by the removable riser cap for conforming to surface variations in the shape of the manifold riser to provide a watertight connection between the riser and the removable riser cap.

[0017] In a further advantageous embodiment, a pressure release valve is disposed within the removable riser cap for venting built-up gases from the hollow chamber. Additionally, a vacuum ventilation valve may also be disposed within the removable riser cap for vacuuming out any excess gases formed in the hollow chamber before attempting to remove the removable riser cap.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The construction designed to carry out the invention will hereinafter be described, together with other features thereof. The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein:

[0019]FIG. 1 is a perspective view of the manhole unit according to the invention;

[0020]FIG. 2 is a bottom view of the manhole unit according to the invention;

[0021]FIG. 3 is a top view of the manhole unit according to the invention;

[0022]FIG. 4 is an exploded view of the manifold inlet connector according to the invention;

[0023]FIG. 5 is a top view of the removable riser cap according to the invention;

[0024]FIG. 6 is a side view of the removable riser cap according to the invention;

[0025]FIG. 7 is a cross-section view of the removable riser cap according to the invention;

[0026]FIG. 8 is a top view of a manhole unit having a plurality of manifold inlets according to the invention; and

[0027]FIG. 9 is a perspective view of the manhole unit connected to a riser extension for raising the manifold riser to ground level according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0028] Referring now to the drawings, the invention will be described in more detail. Referring to FIG. 1, a single-piece corrosion resistant manhole unit, designated generally as A, is shown for placement underground to connect with a conduit system in a watertight manner and provide access to the conduits for inspection, maintenance, and repair. For purposes of describing the preferred embodiment, the conduit system is a series of sewer pipes transferring waste fluid to a treatment facility. The invention is not limited to sewage systems and may be used in to gain access to pipe interiors for a variety of closed fluid transfer systems.

[0029] The manhole is formed as a single piece unit, preferably through a rotational molding process. In the preferred embodiment, the material used to construct the manhole unit is a polyolefin plastic resin, most preferably polyethylene. Polyethylene is preferred because of its extremely strong and durable nature, and its ability to resist corrosive agents such as acids and corrosive gases such as methane and hydrogen sulfate.

[0030] As shown best in FIG. 1, the rotationally molded polyethylene manhole unit includes a dome shaped manifold, designated generally as 10, which forms a hollow chamber with various apertures for connecting to the sewer pipelines. The hollow chamber provides quick and convenient access to the pipe interiors for inspection, maintenance and repair. While the preferred embodiment of the manifold is of a domed shape as shown in FIG. 1, the manifold may be formed to any desired size and shape, depending on the amount of access required by people and equipment.

[0031] Manifold 10 includes a plurality of lateral manifold extensions molded into the manifold for connecting to the sewer pipes. The manifold extensions include at least one manifold inlet connector for connecting to an upstream pipe, and a manifold outlet connector for connecting to a downstream pipe. In the preferred embodiment, the manifold inlet connector is a bell inlet connector 12 for connecting the manifold to an upstream conduit spigot 13 (FIG. 2) for allowing an inflow of fluid to the hollow chamber. The manifold outlet connector is a spigot outlet connector 14, typically disposed on an opposite side of manifold 10 from bell inlet connector 12. Spigot outlet connector 14 allows the manifold to connect to a downstream conduit bell 15 (FIG. 2) for allowing fluid to flow out of the hollow chamber. It is commonly used and known in the art that each section of conduit or pipe be formed with a bell connector end and a spigot connector end. Each conduit bell connecting end receiving the spigot end from the preceding section of pipe. Each conduit bell 15 incorporates a seal member for forming a watertight seal between the conduit bell and preceding conduit spigot end. As best shown in FIG. 2, by incorporating the same type of bell connector with a watertight seal member into the manifold and the standard spigot outlet connector, the manhole unit can be easily and quickly installed without the need for adaptor pipe sections and external sleeve seals that wrap around the outside of the pipes, preventing unnecessary exposure of the seal to corrosive agents surrounding the pipes.

[0032] In the preferred embodiment shown in FIG. 1, the manifold also includes a manifold riser 16, preferably molded into the top of manifold 10. The riser extends in an upward direction from the manifold towards ground level for allowing equipment and personnel to access the hollow chamber when buried several feet underground. Because the manhole unit is molded, the riser can be formed to any height and is not limited to a specific height range. Once the manhole unit is installed and connected to conduits through the bell inlet connector and the spigot outlet connector, it is necessary to provide a removable riser cap 18 to cover and seal the riser providing access to the hollow chamber. This provides a completely enclosed manhole unit that prevents debris and fluids from entering or leaving the manhole, thereby reducing the possibility of overflow and surcharge of the sewage system.

[0033] Because the manifold is intended to be buried underground, often times deep underground, the manifold is designed with a plurality of manifold reinforcing elements that increase the structural integrity of the manifold. In the preferred embodiment, the reinforcing elements are a plurality of reinforcing ribs protruding from the domed sides 22 and base 26 of the manifold. Reinforcing ribs 20 are equally spaced around the domes sides 22 of the manifold. As shown best in FIG. 2, additional reinforcing ribs 24 are shown protruding from base 26 of manifold 10. Together, reinforcing ribs 20 and 24 substantially increase the structural rigidity of the manifold, allowing it to be buried deep underground. Reinforcing ribs 20 and 24 are formed into the sides and base of the manifold during the rotational molding process. As such, the reinforcing ribs are fully integrated into the manifold retaining its single piece construction.

[0034] In the preferred embodiment, a fluid channel 28 is disposed in the hollow chamber of manifold 10 for channeling fluid through the hollow chamber from bell inlet connector 12 to spigot outlet connector 14, as best shown in FIG. 3. Fluid channel 28 is formed as a recess on the interior side 30 of manifold base 26 for channeling water through the hollow chamber, designated generally as 32, from bell inlet connector 12 to spigot outlet connector 14. Preferably fluid channel 28 is declined to form a downward gradient through the hollow chamber from the bell inlet connector to the spigot outlet connector so that fluid is caused to flow through the hollow chamber along the fluid channel and into the downstream conduit. In the preferred embodiment, the fluid channel has a total downward fall of approximately one inch. As shown in FIG. 2, because fluid channel 28 is recessed into base 26 of manifold 10, it protrudes from the base along with reinforcing ribs 24. The reinforcing ribs extend from the side of the protruding fluid channel to approximately bottom edge 34 of manifold 10.

[0035] As shown in FIG. 8, the manhole unit can be rotationally molded so that the manifold includes a plurality of bell inlet connectors 12 a, 12 b, and 12 c for connecting the manifold to a plurality of upstream conduit spigots and allowing an inflow of fluid into the hollow chamber. Accordingly, fluid channels 28 a, 28 b, and 28 c extend between the plurality of bell inlet connectors and spigot outlet connector 14 to channel the fluid through the hollow chamber of the manifold.

[0036] Referring now to FIG. 4, in the preferred embodiment, bell inlet connector 12 includes an annular seal 36 for conforming to exterior surface variations in the conduit spigot to be inserted into the bell inlet connector. Annular seal 36 forms a watertight connection between interior side 38 of bell inlet connector 12 and the exterior side of the conduit spigot, as shown in FIG. 2. Annular seal 36 is carried on interior side 38 so that the annular seal is not exposed to environmental corrosive agents disposed around the outside of the conduits. As such, the seal is less likely to fail than traditional rubber sleeves and will maintain the integrity of the conduit system for extended periods of time. Preferably, the annular seal is an o-ring type seal of the type commonly used in the bell connecting ends of the conduits.

[0037] A seating groove 40 is molded into interior side 38 of the bell inlet connector for receiving annular seal 36. The seating groove holds the seal in position and restricts the seal from moving when the upstream conduit spigot 13 is slid into the bell inlet connector and contacts the seal to form the watertight connection. The conduit spigot will push the annular seal into seating groove 40 and form the watertight connection between the exterior side of the upstream conduit and the interior side of the bell inlet connector.

[0038] In the preferred embodiment shown in FIG. 1, spigot outlet connector 14 includes a beveled spigot edge 42 at distal end 44 of the spigot outlet connector. The beveled spigot edge reduces the thickness of the distal end of the spigot outlet connector for first engaging the seal member carried by the downstream conduit bell. This allows for the distal end of the spigot connector to easily be inserted through the seal member so that the seal member slides up and over the beveled spigot edge and moves into position around the spigot outlet connector to form a watertight connection. As discussed above, because the seal member is disposed on the interior side of the conduit bell, it is not exposed to the environmental corrosive agents that effect traditional seals wrapping around the outside of the connection point.

[0039] Referring to FIG. 9, a riser extension 46 is shown connected with manifold riser 16 to extend the height of the riser to ground level when buried deep under ground so that the hollow chamber can still be easily accessed from ground level when the manifold riser is of insufficient height. Riser extension 46 also includes a sealing ring 48 for forming a watertight connection between manifold riser 16 and the riser extension. The sealing ring is carried on interior side 50 of the riser extension and is held in place by a riser seating groove 52. As with the annular seal carried by the bell inlet connector, the sealing ring of the riser extension is disposed between the interior side of the riser extension and the exterior side of manifold riser 16 so that the sealing ring is not exposed to environmental corrosive agents surrounding the conduits. Preferably, the sealing ring used by the riser extension is a hollow deformable o-ring that compresses against the outside surface of the riser and conforms to the surface shape of the manifold riser to provide a watertight connection.

[0040] As it can be difficult to insert the manifold riser into the riser extension because of the protrusion of the sealing ring, the riser includes a beveled riser edge 54. The beveled riser edge reduces the thickness of distal end 56 of the riser which engages the sealing ring. This allows the distal end of the riser to easily be inserted between the sealing ring, and-as the riser extension is slid over the riser, the sealing ring slides over the beveled riser edge and moves into position around the sides of the manifold riser to form a watertight connection.

[0041] Referring to FIGS. 5, 6, and 7, the removable riser cap will now be described in more detail. As shown in FIG. 5, the riser cap includes a plurality of recessed handles 54 for removing the riser cap from the riser. In order to prevent unauthorized tampering with the manhole unit, handles 54 are removable, and once removed, the riser cap is extremely difficult to take off. As best shown in FIG. 6, removable riser cap 18 includes a pressure release valve 56 for venting any built-up gases within the hollow chamber of the manifold. Pressure release valve 56 may be combined with or replaced by a vacuum ventilation valve capable of allowing a vacuum mechanism to vacuum out an gases formed in the hollow chamber before attempting to remove the riser cap. As shown in FIGS. 6 and 7, removable riser cap 18 also includes an adaptive seal 58 for adjusting to surface variations in the shape of the interior surface of the manifold riser to provide a watertight connection between the riser and the removable riser cap. As with the sealing ring for riser extension 46, the adaptive seal for the removable riser cap is preferably a hollow deformable o-ring capable of compressing against the interior side of the riser into which the cap is inserted so that the o-ring conforms to the interior surface shape of the manifold riser and provides a watertight connection.

[0042] While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims. 

What is claimed is:
 1. A single-piece corrosion resistant manhole unit for placement underground to connect with a conduit system in a watertight manner and provide access to the conduits for inspection, maintenance, and repair, said manhole comprising: a rotationally molded polyethylene manifold forming a hollow chamber for providing access to conduits to be connected to said manifold; a plurality of lateral manifold extensions molded into said manifold for connecting to said conduits to facilitate the flow of fluids through the hollow chamber; at least one manifold inlet connector included in said manifold extensions for connecting the manifold to an upstream conduit spigot and allowing an inflow of fluid to said hollow chamber; a manifold outlet connector included in said manifold extensions for connecting the manifold to a downstream conduit bell and allowing fluid to flow out of said hollow chamber; and, a manifold riser molded atop said manifold and extending upward from said manifold towards ground level for allowing equipment and personnel to access said hollow chamber; whereby a cost effective, lightweight, and corrosion resistant manhole is provided that can be quickly and easily installed with watertight seals to reduce the problems of overflow and surcharge associated with leaking conduits.
 2. The manhole unit of claim 1 including a plurality of reinforcing ribs molded into said manifold; said reinforcing ribs protruding from the manifold to increase the structural integrity of the manifold to withstand external pressures applied to the manifold when buried underground.
 3. The manhole unit of claim 1 including a fluid channel molded into said manifold between said manifold inlet and outlet connectors for channeling fluid through said hollow chamber.
 4. The manhole unit of claim 3 wherein said fluid channel is declined to form a downward gradient through said hollow chamber from said inlet connector to said outlet connector so that fluid is caused to flow through the hollow chamber in the fluid channel and continues through the conduit system.
 5. The manhole unit of claim 1 wherein said manifold inlet connector includes an annular seal for sealing between said manifold inlet connector and said upstream conduit spigot; said annular seal conforming to surface variations in the shape of said conduit spigot to form a watertight connection;
 6. The manhole unit of claim 5 wherein said annular seal is carried on an interior side of said manifold inlet connector so that the annular seal is protected from exposure to environmental corrosive agents surrounding the conduits.
 7. The manhole unit of claim 6 including a seating groove molded into said interior side of said manifold inlet connector for receiving said annular seal; said seating groove restricting the seal from moving when said downstream conduit spigot is slid into said manifold inlet connector and contacts the seal to form the watertight connection.
 8. The manhole unit of claim 1 wherein said manifold outlet connector includes a beveled spigot edge at a distal end of said outlet connector; said beveled spigot edge reducing the thickness of the distal end of the outlet connector for engaging a seal member carried by said downstream conduit bell and allowing the seal member to easily slide over said beveled spigot edge to move into position around the outlet connector and form a watertight connection.
 9. The manhole unit of claim 1 including a riser extension for connecting with said manifold riser; said riser extension receiving said manifold riser to extending the height of said manifold riser to ground level when buried underground, so that access to said hollow chamber can be maintained at ground level when the manifold riser is of insufficient height to reach ground level.
 10. The manhole unit of claim 9 including a sealing ring carried by said riser extension for sealing between said manifold riser and said riser extension; said sealing ring conforming to surface variation in the shape of said riser to provide a watertight connection between said manifold riser and said riser extension.
 11. The manhole unit of claim 10 wherein said sealing ring is carried on an interior side of said riser extension so that the sealing ring is protected from exposure to environmental corrosive agents surrounding the riser extension.
 12. The manhole unit of claim 10 wherein said sealing ring includes a hollow deformable o-ring for compressing against said manifold riser and forming to the surface shape of said manifold riser to provide the watertight connection.
 13. The manhole unit of claim 10 wherein said manifold riser includes a beveled riser edge at a distal end of said manifold riser; said beveled riser edge reducing the thickness of the distal end of the riser for engaging said sealing ring and allowing the sealing ring to easily slide over said beveled riser edge to move into position around the riser and form a watertight connection.
 14. The manhole unit of claim 1 including a removable riser cap for sealing the manifold riser closed to prevent fluid and debris from entering or leaving the manifold.
 15. The manhole unit of claim 14 including an adaptive seal carried by said removable riser cap for sealing between said cap and said manifold riser; said adaptive seal conforming to surface variation in the shape of said riser to provide a watertight connection between said manifold riser and said cap.
 16. The manhole unit of claim 15 wherein said adaptive seal includes a hollow deformable o-ring for compressing against said manifold riser and conforming to the surface shape of the manifold riser to provide the watertight connection.
 17. The manhole unit of claim 14 including a pressure release valve disposed within said removable riser cap for venting built-up gases from said hollow chamber.
 18. The manhole unit of claim 14 including a vacuum ventilation valve disposed within said removable riser cap for allowing excess gases formed in said hollow chamber to be vacuumed out before attempting to remove said removable riser cap.
 19. A corrosion resistant manhole unit for placement underground to connect with conduits of a sewage system, and the like, in a watertight manner and provide access to the conduits for inspection, maintenance, and repair, said manhole comprising: a molded plastic manifold having a hollow chamber for providing access to sewage system conduits to be connected to said manifold; a bell inlet connector extending laterally from said manifold for connecting the manifold to an upstream sewage conduit to facilitate the flow of sewage into said manifold; an annular seal carried by said bell inlet connector for forming a watertight seal between said bell and said upstream sewage conduit; a spigot outlet connector extending laterally from said manifold for connecting the manifold to a downstream sewage conduit to allow sewage to flow out of the manifold; and, a fluid channel molded into said manifold between said bell inlet and spigot outlet connectors for channeling sewage through said hollow chamber; whereby a cost effective, lightweight, and corrosion resistant manhole is provided that can be quickly and easily installed with watertight seals to reduce the problems of overflow and surcharge associated with leaking conduits.
 20. The manhole unit of claim 19 wherein said fluid channel is declined to form a downward gradient through said hollow chamber from said bell inlet connector to said spigot outlet connector so that fluid is caused to flow through the hollow chamber in the fluid channel and continues through the sewage system.
 21. The manhole unit of claim 19 wherein said annular seal is carried on an interior side of said bell so that the annular seal is protected from exposure to environmental corrosive agents surrounding the sewage system conduits.
 22. The manhole unit of claim 21 including a seating groove molded into said interior side of said bell inlet connector for receiving said annular seal; said seating groove restricting the seal from moving when said upstream sewage conduit is slid into said bell and contacts the seal to for a watertight connection.
 23. The manhole unit of claim 19 wherein said spigot outlet connector includes a beveled spigot edge at a distal end of said spigot; said beveled spigot edge reducing the thickness of the distal end of the outlet connector for engaging a seal member carried by said downstream conduit and allowing the seal member to easily slide over said beveled spigot edge to move into position around the spigot and form a watertight connection.
 24. A corrosion resistant manhole unit for placement underground to connect with a conduit system in a watertight manner and provide access to the conduits for inspection, maintenance, and repair, said manhole comprising: a molded plastic manifold forming a hollow chamber for providing access to conduits to be connected to said manifold; a manifold inlet connector molded into said manifold for connecting the manifold to an upstream conduit and allowing an inflow of fluid to said hollow chamber; a manifold outlet connector molded into said manifold for connecting the manifold to a downstream conduit so that fluid flows out of said hollow chamber; a manifold riser carried atop said manifold having an open top forming an aperture for accessing said hollow chamber, said manifold riser extending upward from said manifold towards ground level for allowing equipment and personnel to access said hollow chamber through said aperture; and, a removable riser cap for sealing the top of said riser to close and prevent fluid and debris from entering or leaving the manifold; whereby a cost effective, lightweight, and corrosion resistant manhole is provided that can be quickly and easily installed with watertight seals to reduce the problems of overflow and surcharge associated with leaking conduits.
 25. The manhole unit of claim 24 including a riser extension for connecting with said manifold riser; said riser extension receiving said manifold riser to extending the height of said manifold riser to ground level when buried underground, so that access to said hollow chamber can be maintained at ground level when the manifold riser is of insufficient height to reach ground level.
 26. The manhole unit of claim 25 including a sealing ring carried by said riser extension for sealing between said manifold riser and said riser extension; said sealing ring conforming to surface variation in the shape of said riser to provide a watertight connection between said manifold riser and said riser extension.
 27. The manhole unit of claim 26 wherein said sealing ring is carried on an interior side of said riser extension so that the sealing ring is protected from exposure to environmental corrosive agents surrounding the riser extension.
 28. The manhole unit of claim 26 wherein said sealing ring includes a hollow deformable o-ring for compressing against said manifold riser and forming to the surface shape of said manifold riser to provide the watertight connection.
 29. The manhole unit of claim 26 wherein said manifold riser includes a beveled riser edge at a distal end of said manifold riser; said beveled riser edge reducing the thickness of the distal end of the riser for engaging said sealing ring and allowing the sealing ring to easily slide over said beveled riser edge to move into position around the riser and form a watertight connection.
 30. The manhole unit of claim 24 wherein said manifold riser is molded with said manifold as a single unit.
 31. The manhole unit of claim 24 including an adaptive seal carried by said removable riser cap for sealing between said cap and said manifold riser; said adaptive seal conforming to surface variation in the shape of said riser to provide a watertight connection between said manifold riser and said cap.
 32. The manhole unit of claim 31 wherein said adaptive seal includes a hollow deformable o-ring for compressing against said manifold riser and conforming to the surface shape of the manifold riser to provide the watertight connection. 